1. Field of the Invention
The present invention relates to a hydraulic control device for an automatic transmission.
2. Description of the Prior Art
An automatic transmission has a transmission mechanism that is comprised of rotational elements such as planetary gear mechanisms and friction elements such as clutches and brakes that engage and disengage the rotational elements. The rotational elements are each-actuated by oil pressure, and engage and disengage the friction elements in predetermined combinations to achieve a plurality of gear positions.
FIG. 11 shows an example in which an oil pressure is supplied to a clutch as a friction element. As shown in FIG. 11, a line pressure supplied from an oil pump, not shown, is regulated by a regulating valve 10 under the control of a solenoid 11 and is then supplied to a clutch 40.
The regulating valve 10 receives a control pressure from the solenoid 11 at one control end thereof, and feeds back an output pressure to the other control end to maintain the output pressure corresponding to the control pressure.
An oil pressure switch 13 is provided in an oil channel 42 between the regulating valve 10 and the clutch 40. The oil pressure switch 13 detects the operational timing of the clutch 40 according to a clutch pressure in the process of oil pressure filling or draining in the cultch 40. Oil pressures between interconnecting friction elements are controlled in the detected operational timing so as to realize quick and smooth gear shift.
If oil pressure is supplied against the intention to drain oil pressure or is drained against the intention to supply oil pressure due to a failure of a hydraulic control device, clutches or brakes are unintendedly engaged or disengaged to be brought into a dangerous state such as an interlocked state or a neutral state. To avoid this problem, the oil pressure switch 13 immediately and accurately detects the supply or drain of the oil pressure.
When gears are not changed after completion of gear shift to a gear position at which the clutch 40 is engaged, the regulating valve 10 receives the control pressure directing the maximum command pressure higher than the line pressure from the solenoid 11, and outputs the output pressure corresponding to the line pressure to the clutch 40 to keep the clutch 40 engaged.
Namely, the line pressure is directly supplied to the clutch 40 without being lowered at all.
As shown in FIGS. 12A and 12B showing the basic construction of the regulating valve 10, a spool 35 is inserted into a valve hole 31 comprised of an input port 32, output port 33, and drain port 34 such that the spool 35 is capable of stroking. The line pressure is supplied to the input port 32, and the output port 33 is in communication with the clutch 40.
In a so-called pressure-regulating state in which the command pressure is lower than the line pressure, a low control pressure (SOL pressure) supplied from the solenoid 11 causes a land 36 of the spool 35 to close the input port 32 and form an aperture S1 by a gap between the land 36 and the valve hole 31 and causes a land 37 of the spool 35 to close the drain port 34 and form an aperture S2 by a gap between the land 37 and the valve hole 31 as shown in FIG. 12A. Oil leaked through the aperture S1 raises the oil pressure at the output port 33, and oil leaked through the aperture S2 lowers the oil pressure at the output port 33.
If the oil pressure at the output port 33, which is formed according to balance between the oil leaked through the aperture S1 and the oil leaked through the aperture S2, is higher than the oil pressure corresponding to the SOL pressure, a feed pressure applied from a lower position to an upper position of the spool 35 in FIG. 12A is raised to cause the spool 35 to stroke upward in FIG. 12A to narrow the aperture S1 and widen the aperture S2.
Consequently, the oil leaked through the aperture S1 is decreased and the oil leaked through the aperture S2 is increased to lower the oil pressure at the output port 33.
On the other hand, if the oil pressure of the output port 33 is lower than the oil pressure corresponding to the SOL pressure, a feed pressure applied from the lower position to the upper position of the spool 35 in FIG. 12A is lowered to cause the spool 35 to stroke downward in FIG. 12A to widen the aperture S1 and narrow the aperture S2.
Consequently, the oil leaked through the aperture S1 is increased and the oil leaked through the aperture S2 is decreased to raise the oil pressure at the output port 33.
By repeating the above-described operation, the oil pressure at the output port 33 becomes closer to the oil pressure corresponding to the SOL pressure, and thus forms the oil pressure corresponding to the SOL pressure.
As described above, if the regulating valve 10 is held in the pressure-regulating state, the oil pressure through the input port 32 is limited by the aperture S1, and if the oil pressure higher than the balanced oil pressure is formed, the oil is discharged through the drain port 34. Therefore, even if there is a great change in the oil pressure at the input port 32, the oil pressure is transmitted to the output port 33 after the change is reduced.
On the other hand, if the command pressure is higher than the line pressure, i.e. the control pressure from the solenoid 11 is high, the input port 32 is completely opened as shown in FIG. 12B without being narrowed by the land 36, and the drain port 34 is almost closed to prevent the oil pressure from leaking through the drain port 34.
Therefore, if hydraulic vibration occurs dependently on the structure of the oil pump, the hydraulic vibration is directly applied to the oil pressure switch 13.
A mean value of the oil pressure is called an effective pressure because the mean value of the oil pressure including the hydraulic vibration actually determines the torque transmissible capacity of the clutch. In the case of high-frequency hydraulic vibration, even if the effective pressure of the oil pressure applied to the clutch 40 is low, the hydraulic vibration causes an instantaneous oil pressure in excess of a pressure acceptable by the oil pressure switch to be directly applied to the oil pressure switch to cause a failure thereof because the oil pressure switch and peripheral hydraulic circuits thereof are configured to have a high responsiveness.
If the high-frequency oil oscillation is applied to the line pressure and the oil pressure at which the amplitude of the hydraulic vibration is the smallest becomes lower than the oil pressure at which the oil pressure switch is turned off, the oil pressure switch is repeatedly turned on and off. However, the number of times the oil pressure switch is accurately turned on and off is limited in terms of durability, and if the number of times exceeds the allowable number of times, the oil pressure switch cannot be accurately turned on or off. The high-frequency hydraulic vibration causes the oil pressure switch to be turned on and off with a high frequency, and thus, the oil pressure is repeatedly turned on and off a large number of times in a short period of time. This considerably deteriorates the durability of the oil pressure switch 13.